This invention relates to scanning radio receivers and, more particularly, to scanning radio receivers particularly useful on the frequencies assigned to the Public Safety Radio Services as well as other frequencies.
Scanning radio receivers are well known in the prior art and have found particular utility for the reception of radio signals on the frequencies assigned by the United States Federal Communications Commission to the Public Safety Radio Services. In the past, many such radio receivers used crystals as the tuning element to provide the necessary local oscillator signals and required the presence of one crystal for each frequency the receiver was capable of tuning. Examples of such receivers are those shown in U.S. Pat. Nos. 3,531,724 to G. H. Fathauer, 3,665,318 to S. J. Hoffman, et. al., 3,714,585 to R. C. Koch, 3,725,788 to G. H. Fathauer, 3,794,925 to K. Imazeki, 3,801,914 to K. Imazeki, 3,821,651 to G. H. Fathauer, et al., 3,873,924 to G. H. Fathauer, 3,883,808 to J. E. Boone, 3,824,475 to P. W. Pflasterer, and 3,987,400 to G. H. Fathauer. Recently, scanning radio receivers using frequency synthesizing techniques have been provided which eliminated the need for a large number of crystals in radio receivers capable of being tuned to a large number of frequencies. Such receivers are shown in U.S. Pat. Nos. 3,937,972 to S. C. Snell, 3,961,261 to P. W. Pflasterer, 4,000,468 to J. R. Brown, et al., 4,027,251 to G. H. Fathauer, et al., 4,114,103 to P. W. Pflasterer, and 4,123,715 to G. H. Fathauer. There has also been provided by the prior art scanning radio receivers using frequency synthesizing techniques wherein the frequency synthesizing circuitry was controlled by the operation of a processing means such as a microprocessor. Exemplary radio receivers of this last mentioned type are disclosed in U.S. Pat. Nos. 3,962,644 and 4,092,594 both to W. Baker as well as the presently pending United States application Ser. Nos. 847,497 to G. H. Fathauer, et al., 847,566 to G. H. Fathauer, 000,905 of W. L. Williamson, et al., and 001,013 of A. Khan, et al.
Scanning radio receivers making use of microprocessors to control the frequency synthesizing circuitry have many advantages over the scanning radio receivers known prior thereto. One of the foremost of these advantages is the very high degree of flexibility in control which may be included in such a radio. An example is the "search" mode of operation shown in the aforementioned U.S. Pat. Nos. 3,962,644 and 4,092,594 by which the radio may be successively and automatically tuned to adjacent ones of the Public Safety Radio Service frequencies until the receiver arrives at a frequency upon which a signal is received. Such operation was impractical with scanning radio receivers known prior thereto.
The search mode of operation described above, while being a very advantageous feature, has certain disadvantages and problems which had not been recognized and attacked and, as a result, improved systems have been developed. In particular, systems which embody certain improvements which I invented are disclosed in my aforesaid copending application Ser. No. 034,738, filed Apr. 30, 1979 now U.S. Pat. No. 4,270,217, issued May 26, 1981. It is here noted that such systems also include additional improvements made in accordance with this invention and such additional improvements were disclosed in said application because they had been developed prior thereto and were contemplated as being the best modes known for carrying out the invention as claimed in said application.
As disclosed in my aforesaid copending application, circuitry is provided to enable searching of only those frequencies assigned to particular ones of the services. Such circuitry is also operative to permit reception of both amplitude and frequency modulated signals and to permit searching of both a service such as the aircraft band which utilizes amplitude modulation and another service such as the marine band which uses frequency modulation. The circuitry has other features and advantages including a feature in which both public safety frequencies and aircraft communication frequencies may be scanned in an intermixed and arbitrarily selected order.
The features to which the claims of my aforesaid application are directed are thus highly advantageous but they did not provide a solution to another problem with prior receivers which had not been recognized and attacked. In prior receivers, each time the radio was to be tuned to a new frequency, a signal was first generated to indicate to the controller or microprocessor the need to tune to a new frequency and only after the generation of that signal would the controller or microprocessor commence performing the operations required to generate the control signals effective to retune the receiver to the newly desired frequency. This procedure was particularly disadvantageous when the receiver was used either in a search mode of operation as described above or in a scan mode wherein the receiver is sequentially tuned to frequencies previously selected by the user, as this procedure placed a limitation on the rate at which frequencies could be scanned or searched; however, the problem is of greater import during the search mode of operation wherein it may be desired to search a large number of frequencies.
In particular, the time to search or scan any particular frequency in a receiver using a phase-locked-loop frequency synthesizer circuit was at least as long as the sum of three time periods, the time required for the controller or microprocessor to generate the control signals necessary to tune the frequency receiver to the new frequency, the "settling time" for the phase-locked-loop to adjust itself and bring the local oscillator signal to the desired frequency, and the time needed for the receiver circuitry to reliably detect the presence or absence of a received signal above a minimum signal level at the new frequency. Any steps taken to reduce any of these three time periods can be of importance in materially shortening the time required to search a large number of frequencies. The reduction occurs each time a new frequency is tuned so that the shortening of the time required to search a number of frequencies is equal to the reduction in the time period required for any individual frequency times the number of frequencies searched.
Any such reduction in time can be particularly advantageous in receivers such as aforementioned which have the capability of searching all frequencies assigned to a particular service since in some cases, the number of frequencies assigned to a single service may be large. However, in shortening the time required to tune a receiver to a new frequency during the search or scan modes, it must be recognized that in some cases, the user will use the receiver in a manual mode wherein he will either cause it to become tuned to either one of a plurality of preselected frequencies or to a frequency being newly specified by the user. Increasing the search or scan speeds should not be at the expense of eliminating the capability for operation of the receiver in the manual mode.
Moreover, some scanning radio receivers include a priority operation wherein one receiver channel is assigned a priority status and the receiver automatically and periodically checks the frequency of that channel for the presence of a received signal and, if found, places the receiver on that channel. Any effort to shorten the scan or search time should not adversely affect the priority operation.